Here is a list of sites that offer free graphics you can use on your site.
Note that some of these sites and even some specific photos require that you provide attibution (credit) to the original owner and not all of them are free for commercial use. Always read the license and understand the terms and conditions for the images before you use them.
- Flickr: Creative Commons
- WikiMedia -- Free Images
- WikiMedia -- Piblic Domain
- Canadian Tourism Commission -- Photo & Videos Media Centre
- EveryStockPhoto -- Photo Search Engine
- Free Stock Photos -- Directory at PhotoshopSupport.com
- Fotolia -- Free photo of the week
- iStockPhoto -- Free photo of the week
- Public Domain Images
- ShutterStock -- Free photo of the week
Don't forget, you can always use your own photos. Before using them, always be have permission from people, especially from parents of minors which may appear in your images.